This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Borrelia burgdorferi is known to induce the production of inflammatory mediators in a variety of cells. We hypothesized that in this inflammatory milieu neuronal apoptosis may occur, leading to neuroborreliosis. In experiments where B. burgdorferi was co-cultured in vitro with primary microglia, we observed robust release of IL-6 and IL-8, CCL2 (MCP-1), CCL3 (MIP-1[unreadable]), CCL4 (MIP-1[unreadable]) and CCL5 (RANTES), but we detected no induction of microglial apoptosis. In contrast, SH-SY5Y (SY) neuroblastoma cells co-cultured with B. burgdorferi expressed negligible amounts of inflammatory mediators but also remained resistant to apoptosis. When SY cells were co-cultured with microglia and B. burgdorferi, neuronal apoptosis consistently occurred. Confocal microscopy imaging of these cell cultures stained for apoptosis and with cell type-specific markers confirmed that it was predominantly the SY cells that were dying. Microarray analysis further demonstrated an intense microglia mediated inflammatory response to B. burgdorferi including up-regulation in gene transcripts for proinflammatory cytokines, Toll-like receptor 2 (TLR-2) and NF[unreadable][unreadable]. Interestingly, the pathway that exhibited the most profound changes in regard to inflammatory signaling was triggering receptor expressed on myeloid cells-1 (TREM1). Significant transcript alterations in essential p53 pathway genes also occurred in neuronal cells cultured in the presence of microglia and B. burgdorferi, which indicated a shift from cell survival to preparation for apoptosis when compared to SY cells cultured in the presence of B. burgdorferi alone. These findings indicate that B. burgdorferi is not directly toxic to SY cells;rather, these cells become distressed and die in the inflammatory surroundings generated by microglia through a bystander effect. Such a neuronal impairment may eventually contribute to the neurocognitive symptoms seen in neuroborreliosis.